[MLIR] Printing a null Value.

This diff adds support to printing a Value when it is null. We encounter this situation when debugging the PDL bytcode execution (where a null Value is perfectly valid). Currently, the AsmPrinter crashes (with an assert in a cast) when it encounters such Value.

We follow the same format used in other printed entities (e.g., null attribute).

Reviewed By: mehdi_amini, bondhugula

Differential Revision: https://reviews.llvm.org/D116084